There has been an increase in the use of composite materials in the fabrication of components for aircraft gas turbine engines. Articles made from composite materials offer significant advantages in terms of lower weight and reduced part counts, providing substantial savings in engine assembly, operation and maintenance costs. One article currently under review for a conversion to composite construction is a fan exit guide vane for a gas turbine engine.
Composite fan exit guide vanes have been fabricated and tested against the current vanes made of aluminum, with the composite articles found to be deficient in both fatigue strength and erosion resistance. One method for improving the erosion resistance of composite articles is to include a metal plating on the surface of the composite. In U.S. Pat. Nos. 3,762,835 to Carlson et al, 4,108,572 and 3,572,971 to Seiwert, various metal sheaths, cladding and coatings are discussed for addition to composite articles for improving impact strength and erosion resistance. In U.S. Pat. No. 3,572,971, a titanium cladding is bonded to a composite compressor blade, with the cladding providing erosion protection as well as increased transverse and torsional strength. However, there is no discussion of the fatigue effects involved with these claddings or coatings.
From the experience gained in the plating of metal articles, it is generally known that the addition of a hard plating, such as chromium, may have a detrimental effect on fatigue life. In U.S. Pat. No. 2,732,020 to Scholl, a method for increasing the fatigue strength of metal articles is discussed. While not addressing the fatigue problems with composite articles, it is illustative of the fatigue problems involved with platings. Generally, an aluminum airfoil having a nickel plating electroplated thereon, exhibited fatigue failure after several hours of fatigue testing. By disposing an elastomeric layer between the aluminum substrate and the nickel plating, the fatigue life was increased to fifty hours. However, an intermediate elastomeric layer would be undesirable in many applications, particularly in high temperature service, and may compromise other properties, such as static strength.
In order to utilize composite articles as replacements for metal articles in gas turbine engines, what is needed is a composite article which has erosion resistance equivalent or superior to metal and increased fatigue life.